Directing osteogenic and myogenic differentiation of MSCs: interplay of stiffness and adhesive ligand presentation

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Abstract
The mechanical properties of the extracellular matrix (ECM) can exert significant influence in determining cell fate. Human mesenchymal stem cells (MSCs) grown on substrates with varying stiffness have been shown to express various cell lineage markers, without the use of toxic DNA demethylation agents or complex cocktails of expensive growth factors. Here we investigated the myogenic and osteogenic potential of various polyacrylamide gel substrates that were coated with covalently bound tissue-specific ECM proteins (collagen I, collagen IV, laminin, or fibronectin). The gel-protein substrates were shown to support the growth and proliferation of MSCs in a stiffness-dependent manner. Higher stiffness substrates encouraged up to a 10-fold increase in cell number over lower stiffness gels. There appears to be definitive interplay between substrate stiffness and ECM protein with regard to the expression of both osteogenic and myogenic transcription factors by MSCs. Of the 16 gel-protein combinations investigated, osteogenic differentiation was found to occur significantly only on collagen I-coated gels with the highest modulus gel tested (80 kPa). Myogenic differentiation occurred on all gel-protein combinations that had stiffnesses >9 kPa but to varying extents as ascertained by MyoD1 expression. Peak MyoD1 expression was seen on gels with a modulus of 25 kPa coated in fibronectin, with similar levels of expression observed on 80-kPa collagen I-coated gels. The modulation of myogenic and osteogenic transcription factors by various ECM proteins demonstrates that substrate stiffness alone does not direct stem cell lineage specification. This has important implications in the development of tailored biomaterial systems that more closely mimic the microenvironment found in native tissues.